In the past decade, with the development of low powered devices and appliances, such as the Light-emitting diode (LED), low cost wireless sensors and wireless access points, the self-power technique and the harvesting of energy from ambient vibration have attracted much attention from researchers [1-3]. Among the available mechanic-to-electric energy conversion mechanisms, such as electromagnetic, electrostatic and piezoelectric transductions, the energy density of the piezoelectric transduction is three times higher than electrostatic and electromagnetic transductions [4, 5]. Therefore, energy harvesting by piezoelectric materials has led to many different types of piezoelectric coupled structures for energy harvesting [6˜11]. To further improve the energy harvesting efficiency, different designs of electric circuits and structure optimisation were presented by both numerical simulations and experimental studies. Liao and Sodano [12] studied a single mode energy harvester with different resistances to achieve a larger output electric power. Wang et al [13]proposed an optimal design of a collocated pair of piezoelectric patch actuators surface bonded onto beams. Wang and Wu [14] developed an optimal design of a piezoelectric patch mounted on a beam structure to achieve a higher power-harvesting efficiency through both numerical simulations and experimental studies. Xie et al [15] developed a design of a piezoelectric coupled cantilever structure attached by a mass subjected to base motion to achieve an effective energy harvesting.
In addition, it is noted that there is a huge reservation of sustainable and clear energy on the earth, such as wind and ocean energy. The flowing power of winds is usually from a typical intensity of 0.1˜0.3 kW/m2 to 0.5 kW/m2 on the earth surface along the wind direction, while the flowing power of ocean waves is around 2˜3 kW/m2 under the ocean surface along the direction of the wave propagation [16].
Therefore, based on the direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force), the harvesting of renewable natural energies by piezoelectric materials has been initiated recently to pursue a clean and expedient self-contained energy source. Some research works have been conducted on development of new energy conversion technologies using piezoelectric materials to absorb wind energy, and also flowing water energy in ocean and rivers.
Ovejas and Cuadras [17] developed a wind energy harvester with thin piezoelectric films in a laminar wind tunnel and studied the electric power generation by experiments. Li et al [18] also proposed and tested a bio-inspired piezo-leaf architecture converting wind energy into electric energy by way of a wind-induced fluttering motion. An electric power output of 0.61 mW was generated by the harvester with the dimension of 72×16×0.41 mm. Gao et al. [19] reported a flow energy harvester based on a piezoelectric cantilever (PEC) with a cylindrical extension. This device utilizes flow-induced vibration of the cylindrical extension to directly drive the PEC to vibrate in order to harvest the energy from ambient flows such as wind or water streams. Wu and Wang [20] developed an energy harvester made of a cantilever attached by piezoelectric patches and a proof mass for wind energy harvesting from a cross wind-induced vibration of the cantilever by the electromechanical coupling effect of piezoelectric materials.
From the aforementioned research works, it is found that the harvested electric power from the wind energy is usually low due to the low energy density of the wind flows. In view of considerable large energy density from water flows and wave motions, for example ocean wave motions, which can easily exceed 50 kW per meter of wave front [21], harvesting energy from water flows and waves to generate electric energy by piezoelectric effects has long been pursued as an alternative or self-contained power source.
An energy harvester using a piezoelectric polymer ‘eel’ to convert the mechanical flow energy, available in oceans and rivers, to electric power was presented by Taylor et al [22]. Zurkinden et al. [23] designed a piezoelectric polymer wave energy harvester from wave motions at a characteristic wave frequency and investigated the influences on generated energy from the free surface wave, the fluid-structure-interaction, the mechanical energy input to the piezoelectric material, and the electric power output using an equivalent open circuit model. Xie et al. [24] developed a piezoelectric coupled plate structure, which is fixed on the sea bed, to harvest the horizontal ocean wave energy. Burns [25] provided a piezoelectric device consisted of a buoy floating on the ocean surface, a few anchor chains fixed on the ocean-bed and an array of piezoelectric micro thin films between the buoy and chains, and showed that the device can generate electric power when the piezoelectric films bear tension and compression alternatively duo to the up and down motion of the buoy. Murray and Rastegar [21] presented a novel class of two-stage electric energy generators on buoyant structures. These generators use the interaction between the buoy and sea wave as a low-speed input to a primary system, which, in turn, to successively excite an array of vibratory elements (secondary system) into resonance.
From previous studies, it was proven that energy harvesting from ocean waves by piezoelectric materials is effective and is able to generate sufficient electric power for small electric appliances [24]. However, most of piezoelectric energy harvesting structures in current studies are designed to be fixed on the sea bed, and hence are mostly applicable to shallow ocean and costly. It is obvious that the amount of the ocean wave energy in the intermediate and deep ocean with larger wave heights is much larger than the one in the shallow-water.
Therefore, an urgent need for a more efficient and economical energy harvesting from intermediate and deep oceans calls for challenging engineering designs. Disclosed herein is an expedient and economical floating buoyant energy harvester is developed for energy harvesting from the intermediate and deep ocean waves.